Make:

Before you start playing with the software on the AppleTV, it's prodent to make a bit-level copy of the internal disk. That way, if something goes wrong during your experimentation, you can restore the disk to its original state.

To do this, you'll be removing the AppleTV's disk, putting it in another machine (Linux or OS X), and copying the contents to a file on another hard drive. Note that you'll need a minimum of about 40gig on the other drive to store the disk image. You'll also need a 2.5inch external drive enclosure, or a 2.5inch to 3.5inch IDE adapter for attaching the drive to your other machine. Finally, there's a good chance that opening the case will void your warrantee... but it's for a good cause right?

Back up:

• First, remove the hard disk from your AppleTV. Check out Makezine's instructions to dissect your new toy -Link.
• Connect the drive to your computer as if you were adding a second disk. On Linux, it will be available as /dev/hdb or /dev/hdc depending on whether you slaved it to the first drive or put it on the second channel (we'll assume hdc). In OS X, use the diskutil list command to list the partition tables on your drives. The AppleTV disk will have HFS partitions named Media and OSBoot. We'll assume /dev/disk2 for this article.
• Copy the disk's entire contents to your hard drive. In Linux, run:

  dd if=/dev/hdc of=/home/[username]/appletvdump.img bs=1024k

  For OS X users, it's the same thing, just a different device:

  dd if=/dev/disk2 of=/Users/[username]/appletvdump.img bs=1024k
  

This should take a while, and you'll see/hear both hard drives being accessed. When it's finished, you should have a 40gig (approximately) file on your other computer. On a new AppleTV disk, a lot of this is unused, zeroed out space, so you can gzip it to save some space on your backup machine.

How To Restore:
To restore, we just go through the same process as above, but instead of writing the disk's contents to a file, we write the backup file's contents back to the disk:

In Linux:
dd if=/home/[username]/appletvdump.img of=/dev/hdc bs=1024k
In OS X:
dd if=/Users/[username]/appletvdump.img of=/dev/disk2 bs=1024k

That's all there is to it. With your backup in place, you're now free to monkey around and try some of the other tricks we're posting on this AppleTV weekend (or concoct a hack or two of your own)!